Solvents and solvent effect in UV - Vis Spectroscopy, By Dr. Umesh Kumar sharma and Susan Jacob

SOLVENTS AND EFFECT OF SOLVENTS IN UV - VIS SPECTROSCOPY, By Dr. UMESH KUMAR SHARMA & SUSAN JACOB

1. 1 By – Dr. UMESH KUMAR SHARMA & SUSAN JACOB DEPARTMENT OF PHARMACEUTICS MAR DIOSCORUS COLLEGE OF PHARMACY, THIRUVANANTHAPURAM , KERALA, INDI A UV-VISIBLE SPECTROSCOPY SOLVENT AND SOLVENT EFFECT

2. The absorption bands in ultraviolet spectrum are very broad as compared with infrared or NMR spectrum. U ltraviolet spectra of compounds are usually determined either in the vapour phase or in very dilute solution. S olvent must be transparent with in the wavelength range being examined. 2 CHOICE OF SOLVENT

3. It should not itself absorb radiation in the region under investigation It should be less polar, to minimum interaction with solute molecule. Spectrascopic (Analytical) grade solvents should be used. The most commonly used solvent is 95% ethanol. It Is a cheap Good dissolving power Does not absorb radiation above 210nm Some other solvents which are transparent above 210 nm. Eg : n-hexane , cyclohexane, methanol, water and ether. 3 A solvent for UV spectroscopy should meet the following requirements

4. Hexane and other hydrocarbons are sometimes preferred to polar solvents. Because they have minimum interaction with solute molecule. Benzene, chloroform, carbon tetrachloride can not be used. Because they absorb in the range of about 240 - 280nm. Generally polar solvents such as water, alcohol, esters and ketones tend to destroy the fine structure due to vibrational effect. 4

5. SOLVENT LOWER WAVELENGTH LIMIT /nm Water 205nm Ethanol 210nm Hexane 210nm Cyclohexane 210nm Chloroform 245nm Carbon tetrachloride 265nm Benzene 280nm 5 Solvents for electronic spectroscopy

6. Solvatochromism : It is the ability of a chemical substance to change colour due to c hange in solvent polarity. The sign of solvatochromism depends on the difference in the dipole moment of the ground and excited state of the chromophore . The presence of specific and nonspecific interaction between the solvent and solute molecule are responsible for the change in molecular geometry, electronic structure and dipole moment of solute. These solute - solvent interactions affect the solute electronic absorption spectrum . This phenomenon is known as Solvatochromism . 6 SOLVENT EFFECT

7. Chromophore : Covalently bonded unsaturated group capable of absorption of radiation. Chromophore may have π electrons or n electrons which may undergo π → π * and n → π * transitions. Chromophore was applied to the system, responsible for imparting colour to the compound. Eg : E thylenic , A cetylenic , Carbonyls, Acids, Esters etc 7

8. Auxochrome Auxochromes are fully saturated groups with lone pair of electrons. Auxochrome when attached to the chromophore , shift the absorption peak towards the longer wavelength & also increase the intensity of absorption. Eg : -OH, -NH 2 , -OR, etc 8

9. Bathochromic shift Hypsochromic shift Hyperchromic shift Hypochromic shift 9 Types of solvatochromatic shifts

10. 1 . Bathochromic shift A shift of absorption peak towards longer wavelength is called bathochromic shift or red shift or positive solvatochromism . This shift is seen with increased polarity for π - π * transitions. The increased interaction between the solvent and the absorbing species lowers the energy level of excited and unexcited state. But the effect is seen more on the excited state. 10 π * π π *orbital lowered by solvent interaction π orbital lowered by solvent interaction

11. Alkyl substitution of olefins shift the absorption maxima to longer wavelength , the shift increases as number of alkyl group increases. Ethylene shows absorption 170nm where as 1,3-butadiene shows at 217nm. The bathochromic shift is progressive as the number of alkyl group increases. Conjugation shift the absorption maxima to longer wavelength. This is because delocalisation of π electrons . Which lower the energy of π * orbital giving it less anti-bonding character. Eg : Conjugation between the doubly bonded oxygen of aldehyde, ketones , carboxylic acid and olefinic double bond. 11 Examples

12. Red shift is seen for phenol when converted to phenolate ion in solution. Since an extra lone pair of electrons are available for interaction with π electrons of ring. In alkaline medium, p-nitro phenol shows red shift. Because negatively charged oxygen delocalizes more effectively than the unshared pair of electrons . There is a bathochromic shift of 240 nm in the λ max of benzene due to presence of auxochrome NH 2. A Bathochromic shift of n- π * band is observed in carbonyl compounds on decreasing solvent polarity. Eg : λ max of acetone is at 264.5 nm in water as compared to 279 nm in hexane. 12

13. 2 . Hypsochromic shift / Blue shift Peaks associated with n - π * transitions are generally shifted to shorter wavelength with increasing polarity of solvent. This may be due to increased solvation of un bonded electron pair which lowers the energy of n orbital. When polar solvents like water or alcohol is used, hydrogen bond formation between the solvent protons and the non bonded electron pair lower the n-orbital by a value, approximately equal to energy of hydrogen bond . Hypsochromic effect is also seen when hydrogen atom in aldehyde is replaced by a methyl group. 13

14. π * n orbital shifted to lower n 1 energy by hydrogen bonding 14 293 nm 280 nm

15. Examples Blue shift is seen for aniline in acidic solution due to formation of anilium cation which has no lone pair of electron to interact with π electron of ring. Aniline -280 nm (conjugation of pair of electron with benzene ring). In acidic solution, removal of conjugation or removal of lone pair of electron. The absorption take place at lower wavelength 200 nm 15 λ max = 280 nm λ max = 200nm.

16. 3 . Hyperchromic shift When absorption intensity of a compound is increased, It is known as H yperchromic shift. If auxochrome introduces to the compound, the intensity of absorption increases. Eg : pyridine- 257nm and Ꜫmax is 2750, 2-methyl pyridine 260 nm and Ꜫmax is 3560. Benzene shows B-band at 256nm Ꜫmax is 200 where as aniline shows B-band at 280 nm Ꜫmax is 1430 . The increase of 1230 in the value of Ꜫmax of aniline compared to that of benzene is due to the hyperchromic effect of auxochrome NH 2. 16 Pyridine λ max = 257 nm 2-methyl pyridine λ max = 260 nm

17. 4. Hypochromic shift Decrease in absorption intensity, Ꜫmax is called hypochromic shift. This is caused by the introduction of a group which distorts the chromophore . Eg : Biphenyl shows λ max 252 nm, Ꜫmax 19000, whereas 2,2 dimethyl biphenyl shows max 270 nm, Ꜫmax 800. The decrease of 18200 in the value of Ꜫmax of 2,2dimethyl biphenyl is due to the hyochromatic effect of methyl group which distorts the chromophore by forcing the ring out of co-planarity, resulting in the loss of conjugation . 17 Biphenyl Ꜫ max =19000 2,2 D imethyl Biphenyl Ꜫmax= 800

18. Naphthalene shows intensity of absorption Ꜫmax 19000 and 2-methyl naphthalene shows Ꜫmax 10250. 18

19. The absorption exhibited by a single stranded DNA is higher than the absorption exhibited by a double stranded DNA. Single s tandard DNA- hyperchromic effect- bases are in free form. DS DNA- hypochromic effect decrease resonance behavior & decrease UV absorbance. 19 DNA –Effects of hyperchromicity and hypochromicity

20. Effects of solvent polarity on various types of bands K band : The conjugated enes and enones are affected differently by changing the polarity of the solvent. In K bands the conjugated dienes are not affected by changing the polarity of the solvent while enones shows a red shift by increasing polarity of solvent. R band : The absorption shift to shorter wavelength (blue shift) with increasing polarity of the solvent. B band : The position as well as the intensity of the band is not shifted by increasing the polarity of solvent. But the heterocyclic aromatic compound, a marked hyperchromic shift (increase in Ꜫmax) is observed by increasing the polarity of the solvent. Effects of solvent also effects the fineness of absorption bands in the UV spectrum If the dielectric constant of the solvent is high. There will be stronger solute solvent interaction. Due to this vibrational and rotational energy states of a molecule increase thus fineness of absorption band falls . 20

21. Skoog , Holler, Crouch, Principles of Instrumental Analysis, 6 th edition. William K emp organic chemistry, 3 rd edition, page no. 256-259. B. K. Sharma, Instrumental methods of chemical analysis, page no. S-138. 21 References

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